Traditional microfabrication processes are primarily suited for two-dimensional (2D) designs. These processes have excellent in-plane dimension control due to precise lithography but very limited out-of-plane dimension control. The only out-of-plane dimension control available is the layer thickness or depths of etches. Thus, 2D patterning places constraints on potential device designs and applications.
Smooth, rounded out-of-plane structures are particularly difficult to construct using conventional microfabrication techniques. Grayscale lithography is a common technique employed to fabricate three-dimensional (3D) microstructures. However, the grayscale lithography process is very expensive and suffers from low out-of-plane resolution.
Smooth, rounded microstructures have been fabricated via two different material reflow processes. The first process includes heating a material above its softening temperature, which allows surface molecules of the material to reorganize into a lower surface energy state. As a result, sharp and corner edges are rounded and surface roughness is reduced. The rounded microstructure is maintained when the temperature is lowered, solidifying the reflowed structure.
The second process also includes heating a material above its softening temperature to reflow it in-to or out-of cavities etched in a handle wafer. The direction of reflow depends on the differential pressure applied across the material. The differential pressure is controlled by varying the chamber pressure when the reflow material is bonded to the handle wafer. Other environmental conditions are used to manipulate the shape of the reflowed material such as reflow temperature, reflow time, material type, and the like. The material is then fixed in a final state by cooling below the softening point. The elevated temperature during the reflow process allows for minimization of the material's surface energy, resulting in rounded corners and edges with minimal surface roughness.
Unfortunately, these reflow processes are limited in the number of material sets that are compatible. For instance, materials with very high softening temperatures are not compatible with the processes above because of the difficulty of finding a handle wafer that survives an extremely high reflow temperature. Furthermore, the reflow process requires materials that are amorphous and inert such that the material retains its chemical composition and material properties during the reflow step.